Various measuring devices for measuring linear distances using one or more laser radars are known. Such measuring devices may generate information related to a distance or range of a target from the measuring device and/or a velocity, or range rate, of the target relative to the measuring device. This range and range rate information may be useful in a variety of settings. For the purposes of this application the term range rate refers to the rate of change in the range between the target and the measuring device.
A typical measuring device may include, for example, a frequency modulated laser radar system. The system may include a laser source that emits a beam of electromagnetic radiation. The beam may be emitted at a frequency that is continuously varied, or chirped. In some instances, chirping the frequency may include sweeping the frequency between a lower frequency and an upper frequency (or vice versa) in a periodic manner (e.g. a sawtooth waveform, a triangle waveform, etc.). The beam may be divided into a target beam and a reference beam.
In conventional embodiments, the system may include a target interferometer and a reference interferometer. The target interferometer may receive the target beam, and may generate a target signal corresponding to a frequency difference between one portion of the target beam directed towards, and reflected from, the target, and another portion of the target beam that directed over a path with a known or otherwise fixed path length. The frequency difference may determined by the target interferometer based on an interference signal derived from the two portions of the target beam. The reference interferometer may receive the reference beam and may generate a reference signal corresponding to a frequency difference between two portions of the reference beam that may be directed over two separate fixed paths with a known path length difference. The frequency difference may be determined by the reference interferometer based on an interference signal derived from the two portions of the reference beam.
Generally, the system may include a processor. The processor may receive the target signal and the reference signal and may process these signals to determine the range between the target interferometer and the target. Range information determined based on the target signal and the reference signal may be used to determine a range rate of the target with respect to the target interferometer.
Conventional systems may be built, for example, as described in U.S. Pat. No. 5,114,226, entitled “3-DIMENSIONAL VISION SYSTEM UTILIZING COHERENT OPTICAL DETECTION,” which is incorporated herein by reference in its entirety.
Conventional systems are typically limited in various aspects of operation. For example, these conventional systems are not able to provide range and/or range rate information instantaneously based on the target signal and reference signal, or unambiguously determine distance and velocity. These conventional systems are limited in other ways as well. These limitations may be exacerbated by various operating conditions such as, for example, target acceleration toward or away from the target interferometer, using an actuated optical element (e.g. a mirror or lens) to scan the target at high speeds, or other operating conditions.
In some configurations, beams produced by two laser sources may be combined to provide a beam of electromagnetic radiation that may then be divided into a reference beam and a target beam. In these configurations, the frequencies of the two laser sources may be counter chirped, or, in other words, the two frequencies may be chirped such that while a frequency of one of the laser sources is ascending toward an upper frequency, the other is descending toward a lower frequency, and vice versa. Systems utilizing such a configuration may suffer some or all of the drawbacks associated with single laser source systems, as well as other drawbacks unique to two laser source systems.